Growth hormone (GH) is an anabolic cytokine hormone important for linear growth in childhood and normal body composition in adults1. The current therapeutic regimen for GH replacement requires once-daily subcutaneous injections which is inconvenient and expensive. A number of approaches have been taken to create long-acting preparations, including pegylation2 and sustained-release formulations3-5. Pegylation has the disadvantage that it reduces affinity of hormone for receptor2, and chemical modification with subsequent purification is expensive. Sustained-release formulations have proven efficacy4-7 but such GH preparations are characterised by a dominant early-release profile, causing supraphysiological GH levels3, manufacture is expensive and injections may be painful4. There is a need for growth hormone formulations that minimise manufacturing costs, have good pharmacokinetic profiles, are easy to administer, and acceptable to patients.
GH acts through a cell-surface type 1 cytokine receptor (GHR). In common with other cytokine receptors, the extracellular domain of the GHR is proteolytically cleaved and circulates as a binding protein (GHBP)8. Under physiological conditions GH is in part bound in the circulation in a 1:1 molar ratio by GHBP and this complex appears to be biologically inactive, protected from clearance and degradation9,10. A cross-linked complex of GH with GHBP has delayed clearance but no biological activity11. Co-administration of separately purified GHBP with GH in a 1:1 ratio can augment the anabolic actions of GH12. Thus, like many hormonal systems, binding in the circulation provides an inactive circulating reservoir in equilibrium with active free hormone13. GH binds sequentially with two membrane bound growth hormone receptors (GHR) via two separate sites on GH referred as site 1 and site 2. Site 1 is a high affinity binding site and site 2 a low affinity site. A single GH molecule binds 1 GHR via site 1. A second GHR is then recruited via site 2 to form a GHR:GH:GHR complex. The complex is then internalised and activates a signal transduction cascade leading to changes in gene expression. The extracellular domain of GHR exists as two linked domains each of approximately 100 amino acids (SD-100), the C-terminal SD-100 domain (b) being closest to the cell surface and the N-terminal SD-100 domain (a) being furthest away. It is a conformational change in these two domains that occurs on hormone binding with the formation of the trimeric complex GHR-GH-GHR.
In our co-pending applications WO01/96565, WO2009/013461 and currently unpublished PCT/GB2008/003056 we disclose growth hormone fusion proteins that behave as antagonists and agonists of growth hormone receptor activation. The fusion of native growth hormone and modified growth hormone to the extracellular domain of growth hormone receptor creates fusion proteins that have improved pharmacokinetics and pharmacodynamics with super agonist and antagonist activity.